1. Field of the Invention
The present invention is directed to the field of pollution control, and is more specifically directed to a system and process for controlling mercury and other pollutant emissions in a cement manufacturing facility.
2. Description of Related Art
The cement manufacturing process begins with quarrying the raw materials. The primary raw material component is limestone which serves as the calcium source for the cement. Smaller quantities of sand (source of silica), shale or clay (source of aluminum), and iron ore or slags (source of iron) are also utilized in the process. The quarried material is reduced in size by a crusher and the crushed material is then transported to the cement manufacturing facility. The proper proportions of the raw materials are then mixed and further reduced by grinding the materials in a raw mill to form the raw mix. This raw mix is heated within a pyroprocessing kiln at relatively high temperatures in excess of 2750° F. to form “clinker.” The clinker is further processed and mixed with gypsum to form cement. The pyroprocessing kiln is fired with large quantities of fuel. A variety of different fuel sources may be utilized, including coal, coke, natural gas, heating oil, tires, and/or waste derived fuels.
The heated exhaust process gas from the kiln contains a significant amount of particulate material known as kiln dust as well as various gaseous pollutants such as organic compounds, carbon dioxide, oxides of sulfur, hydrogen chloride, oxides of nitrogen, and trace metals such as mercury. A majority of the particulate material may be removed from the exhaust process gas and recycled back into the raw mix using various processing techniques known in the industry. Of note, the heated exhaust process gas leaving the kiln may be passed through a pre-heater tower within the pyroprocessing unit and/or may be passed through the raw mill. While the primary purpose for running the gas though these units is to utilize the heated gas to heat and dry the raw mix, a secondary benefit is that a substantial amount of the kiln dust within the gas is captured back into the raw mix for subsequent processing. In addition, it is known to pass the exhaust process gas through a particulate control device such as a baghouse to filter and remove additional kiln dust from the gas before emitting the gas from the facility via a stack. This collected kiln dust may then be mixed with the raw mix and recycled back to the pyroprocessing kiln.
Additional methods have been developed for further reducing the amount of particulates and other pollutants from the emissions of cement manufacturing facilities. For example, U.S. Pat. No. 7,040,891 to Giuliani discloses a system for reducing fuel consumption and pollutant emissions from asphalt production facilities by supplying at least one sorbent to the exhaust process gas stream coming from the kiln and then conveying the treated exhaust process gas to a baghouse in order to precipitate at least a portion of the spent sorbent. A portion of the treated gaseous waste stream from the baghouse is then directed to the inlet burner of the kiln as fuel in order to reduce fuel consumption of the kiln. Any residue or organic compounds and/or carbon monoxide are burned within the kiln helping to reduce the concentration of these compounds in the stack. While this method may be useful for pollution control, it does not enable recycling and reuse of the kiln dust as raw mix for clinker production, since the filtered waste stream is “contaminated” with spent sorbent. U.S. Pat. No. 7,279,039 to Schwab et. al. also discloses a method and apparatus for reducing air pollutants associated with cement manufacturing, wherein the raw mix is heated in a special heating chamber to drive off volatile compounds such as organic materials and salts of ammonia, before introduction into the kiln. While this method may be useful for capturing and controlling the volatile compounds contained within the exhaust process gas, a need remains in the art to further enhance pollutant control and, in particular, to further reduce the emission of vapor phase trace metals such as mercury.
Both the raw materials and the fuel used in the cement manufacturing process may contain minute concentrations of mercury (Hg), which can become part of the stack emissions when heated in the kiln system to the high temperatures necessary to produce quality clinker. Mercury is found in the environment in several forms including elemental (Hg), oxidized (HgO), organic (e.g. CH3Hg, MeHg) and particulate-bound. Limited data are available relating to mercury emissions associated with cement kiln operations, but it is believed that all forms of mercury are found within the emissions except organic mercury which is likely thermodynamically precluded from forming due to the high temperatures involved in the process. Significant variability over short periods of time in both total mass mercury may be found in the exhaust process gas due to the variability in concentrates and type of mercury found in the various raw materials and fuel.
A variety of techniques have been used for removing mercury from gas streams in other industries such as from the flue gas of coal fired power plants. These techniques include injecting sorbents into the gas stream before a particulate collection device, passing the gas stream through a packed bed of sorbent or using a wet system to capture mercury compounds. However, these various techniques have not heretofore been employed for removing mercury from the exhaust process gas of a cement manufacturing facility, wherein the chemical content of the exhaust process gas and the components of the facility are very different from a power plant.